Air flow control for bowl mills



\N. R. WOOD ET AL AIR FLOW CONTROL FOR BOWL MILLS April 1, 1941.

3 Sheets-Sheet l 1 J 5 a AM i w y F U v |V| I] w 1 H| l 0v 9 I W 1 mm 1 Z 1 Y 1 3 4 I 1 Filed Oct. 8, 1958 Inxfanfirs Qfood 2 23 22s 2 M and 8d 3 orfigis April 1, 1941. w. R. WOOD ETAL AIR FLOW CONTROL FOR BOWL MILLS Filed Oct. 8, 1938 3 Sheets-Sheet 2 I a WW April 1, 1941. w. R. WOOD EIAL AIR FLOW CONTROLFOR BOWL MILLS Filed Oct. 8, 1938 I5 Sheets-Sheet 3 lli l I 4y 40 7114/, l

W21 %'ngnvvr.s 4 ,1 fed 00d 0 d ui'f 752228 e/df I (B M Onngys Patented Apr. 1, 1941 AIR. FLOW CONTROL FOR BOWL MILLS Wilfred R. Wood, New York, and Kurt Toensl'eldt,

Port Washington, N. Y., assignors to Combustion Engineering Company, Inc., New York, N. Y., a corporation of Delaware Application October 8, 1938, Serial No. 233,902

8 Claims.

This invention relates to certain new and useful improvements in a rotary bowl mill adapted to grind and pulverize solid materials, for example coal or rock, and more particularly to means for controlling and properly distributing the air flow through a mill of this type.

Briefly described, the mill is of the type including an upwardly opening bowl or grinding ring which is rotatable about a central vertical axis within a substantially closed housing or casing. A plurality of grinding elements, for example,

- rollers each freely rotatable about its own axis,

are suspended within the bowl or ring so as to be forced under pressure toward the upwardly and outwardly projecting inner side wall of the bowl, or against the material carried thereby, the rollers being rotated by contact with this material. The side walls of the bowl are spaced from the side walls of the container so as to provide an annular air passage around the bowl, air being admitted to the lower portion of the passage and drawn or forced under pressure through the annular passage and thence over the bowl and through outlets in the upper wall of the casing to a separator. sufficiently pulverized material is carried in suspension in the air stream from the easing into and through the separator. The material to be pulverized is introduced from above into the central portion of the bowl, and insufliciently pulverized material is returned from the separator into the central portion of the bowl. Stationary deflectors are positioned in the casing at the outer side of and above the annular air passage to return some insufficiently pulverized material thrown from the bowl back into the bowl for further treatment. The heaviest waste materials, for example tramp iron, fall through the annular passage into the lower portion of the casing and are discharged from the bottom of the casing. A mill oi the general type described hereinabove is disclosed and claimed in the patent of Joe Crites, 2,079,155, granted May 4, 1937.

In a mill of this type, the upward air flow through the annular passage around the bowl must have at least a certain minimum velocity in order to sustain and carry upwardly the pulverized material spilled from the bowl. Under certain conditions an excessive flow of air is necessary in order to maintain this velocity. For example, if the mill is used for pulverizing coal, and the pulverized coal is carried directly in suspension in the air stream into a furnace, there will be too much primary air for combustion when the mill is operating under a light load. If the air stream is cut down so as not to provide too much combustion air, there will be insufllcient velocity around the bowl to sustain the pulverized coal in the air stream, and the coal will spill over the edge of the bowl and downwardly through the air passage to be discarded. Of course, the width of the annular passage could be restricted thus increasing the velocity for a given volume of air, but this is unsatisfactory for several reasons. The passage will be too narrow to allow pieces of tramp iron or other foreign materials to pass downwardly between the bowl and housing, and there will be an insufficient volume of air to properly handle the pulverized material at those locations where the greatest volume of material is thrownirom the bowl.

According to the present invention means are provided for restricting the annular passage at those locations where only a small volume of air is'necessary, while leaving intermediate portions of the passage unrestricted so as to provide an adequate volume of air at proper velocity, and also leaving adequate space for the discharge of foreign materials from the mi1l. In this way the required air velocity is obtained with a greatly decreased volume of air flow. Specifically, a series of aligned circumferentially extending ribs is positioned on the innersurface of thecasing wall, the ribs extending inwardly adjacent the surface of the rotary bowl. The ribs are preferably positioned back of the rollers where there is the least discharge of material from the bowl, the unrestricted portions of the annular passage, between the ends of the ribs, being located opposite the spaces betweenthe rolls where the maxi-,

mum quantity of material is spilled or thrown from the bowl.

The principal object of this invention is to provide an improved bowl mill of the type briefly described hereinabove and disclosed more in detail in the specification which follows.

Another object is to provide improved means for controlling and properly distributing the upward flow of air around the bowl of a. bowlmill.

Other objects and advantages of this invention willbe more apparent from the following detailed description of certain approved-form of apparatus constructed and operating according to the principles of this invention. p

In the accompanying drawings Fig. 1 is a. centralvertical section through the improved mill assembly.

Fig. 2 is a horizontal section, taken substantially on the line 2-2 of Fig. 1.

Fig. 3 is a partial perspective view of the inner portion of the casing wall at the location of the restricting ribs.

Fig. 4 is a horizontal section similar to Fig. 2, illustrating a two-roller mill, and a somewhat differentform and positioning of the restricting ribs.

Fig. 5 is a vertical section taken substantially on the line 55 of Fig. 4.

Fig. 6 is a partial vertical section taken onv the line 6-6 of Fig. 4. I I

This assembly comprises a casing or housing "A, the rotatable bowl or container B which is driven and supported by the mechanism indicated generally at C, the grinding rollers D, and the mechanisms indicated generally at E for supporting and exerting adjusted pressure on the rollers. The means shown at F feeds material to be ground into the mill, and the conduit system indicated generally at G delivers air under pressure into the lower portion of the mill housing. At H is indicated the mechanism for discharging waste materials from the mill, while at K is shown the separator through which the sufficiently pulverized material is carried out in suspension and from which insufliciently pulverized material is delivered back into the bowl B. At L is indicated generally the improved means for controlling the air-flow through the mill, this openings 8 each closed by an individual cover.

plate 9. .Each opening 8 permits the insertion or removal of one of the roller assemblies D, and the roller supporting and adjusting mechanism E is carried by the cover plate 9. The main cover.

plate 4 is also provided with a plurality of. outlet openings which communicate with passages within conduits Ill leading up into the separator K. A central opening II in cover plate 4 communicates with the lower discharge end of the separator K and will be hereinafter described. The feeding mechanism F also extends through an opening in the wall 4. Except for the openingsreferred to above the top of the casing may be considered as entirely closed.

The bowl or container B comprises a central 'shallow table portion I2 which flares outwardly at a small angle to the horizontal as at I3, and an annular grinding wall formed by an upwardly projecting outer flange portion I8 which flares outwardlyat a small angle to the vertical.

There is an annular passage I5 between wall I8 of the bowl and the side wall 3 of easing A. Preferably a separate grinding'track or ring I6 is mounted on the inner surface of the inclined side wall I4 of the bowl, this ring providing the grinding surface opposed to the. rollers D.

The driving assembly C comprises a central vertical drive shaft I1 which is keyed within the hub 6 of the bowl B. The upper and lower members I8 and IQ of a gear casing are secured together and to the base plate I so as to enclose and lubricate the gears which rotate shaft 11. A worm wheel 20 keyed on shaft I'I meshes with and is driven by a worm 2| which is rotated by a driving motor or other suitable driving means not here shown.

There may be one or more-of the grinding rollers D, depending on the size of the mill. Preferably there will be either two or three of these rollers as in the examples here shown. These rollers will be equally spaced about the central axis so as to distribute the thrust on the grinding bowl 3. Eachroller comprises a removable and renewable grinding tread 22 carried by the sleeve assembly 23 which is freely rotatable on the nonrotary supporting shaft 24. This type of grinding roller is well known in the art. The roller D is rotatable by contact with the grinding ring I6, or more properly by contact with the material to be ground which is interposed between the tread 22 of the roller and the grinding ring as will be hereinafter apparent. Preferably the vertical axis of the bowl, the central axis of rotation of each roller,,and the apex'es of the conical grinding surfaces of the rollers D and the grinding ring I6, all meet in a common point positioned below the bowl so that there will be a true rolling engagement between the surfaces of the rollers and grinding ring.

The roller-supporting mechanism E comprises a lever assembly 25 supporting the upper end of shaft 24 and pivoted at 26. The adjustable spring mechanism 21 tends to swing the roller assembly in a clockwise direction about the axis 26 (as seen at the left of Fig. 1) so as to force the roller D yieldably against the grinding ring I6. As material to. be ground finds its way between roller D and grinding ring I6, the spring 21 will yield to permit the roller to move inwardly, but a predetermined outward pressure will be exerted on the material, this pressure being determined by proper adjustment of the spring mechanism. It will be understood that the above brief description is of one of the roller units D and E, each of these units consisting of an assembly such as just described hereinabove. By removing the individual cover plate 9, each of these grinding units can be lifted from the bowl and housing.

The material to be ground and pulverized is fed into the housing through chute .F, it being understood that this feeding mechanism will also usually include a hopper and rotary feeding mechanism so as to graduate the rate at which material is fed into the mill. This mechanism (not here shown) will also prevent the flow of air through chute F. v

Air from any suitable source is delivered through conduit G tangentially into the lower portion of the mill housing through inlet opening 28. thus causing a whirling movement of the air within the housing. This air is delivered into the housing under pressure, but this pressure may be created by a suction at the outlet 29 of separator K. This air stream passes upwardly through the annular channel I5 between the rotating bowl and the side wall 3 of the casing, thence inwardly over the bowl and out through the passages leading upwardly at I 0 into the upper portion of the separator K.

The separator K may be of any suitable type, but is here shown in the form disclosed and claimed in the patent to Crites, 2,100,734, granted November 30, 1937. Briefly described, the air stream carrying pulverized material in suspension passes upwardly through conduits l into the annular manifold 30 from which it is discharged tangentially with a whirling motion through outlets 3| into the upper portion of separating chamber 32. The larger and heavier particles will separate out and be returned through bottom opening H into the bowl B for further grinding. The sufficiently pulverized material will remain in suspension and be carried upwardly as indicated by the arrows through the outlet conduit 29. This material may then be collected from the air stream by means of a suitable separator, or in the case of pulverized coal it may be carried directly into a furnace in suspension in the air stream.

ing process, the material in bowl B will move by centrifugal force toward the outer wall ll of the bowl and will be ground and pulverized between the rolls D and the grinding ring l6. Due to the small outward inclination of the outer walls of the bowl, centrifugal force will hold a compact bed of the material against the grinding ring, through which bed of material the rollers D grind, and this material will move slowly upwardly before being thrown from or spilled over the upper edge of the bowl. It has been found that the greater portion of this material leaves the bowl over those portions of the bowl edge that are temporarily positioned between successive rollers, there being comparatively little of the material discharged from the bowl radially back of the rollers or at some considerable distance at either side therefrom.

A plurality of stationary deflectors 33 are positioned at spaced intervals circumferentially of the housing in the upper and outer portion therethe form of an annular angle plate such as here 4 shown. Material from the bowl will fill in this angle So as to provide an upwardly and outwardly sloping surface, and material which would otherwise be thrown substantially horizontally by centrifugal force will be directed upwardly and outwardly by this sloping surface toward the stationary deflector vanes 33; Due to the rapid rotation of bowl B this discharged material will be thrown forwardly or tangentially substantially in the direction of rotation of the bowl and at the same time will be directed upwardly so as to contact with the stationary deflectors 33. This material is also projected through the air stream which is moving spirally upward through the annular passage l5, and this air stream will be of sufiicient volume and velocity to carry sufficiently pulverized material in suspension and eventually carry this material upwardly into separator K. Larger particles of material which have not been sufficiently pulverized or reduced in size and which are too heavy to be carried in suspension in the air stream will be thrown back through the air stream by the deflecting vanes 33 into the central portion of bowl B and thus returned to the grinding cycle. The heavier and larger particles of adjacent the side wall 3 .and top wall 4. These of waste material'which cannot be ground in the mill and are to be rejected will be thrown outwardly from the bowl in a more horizontal direction and fall back through passage l5 into the lower chamber beneath the bowl.

The discharge mechanism H comprises a 'discharge opening 36 extending down from chamber 35 through the base plate I and communicating with the discharge chute 31 provided with the one-way valve 38 which minimizes the inflow of air through this chute. A plurality of scrapers 39 are carried by the rotating bowl B, these scrapers moving material deposited in chamber 35 around into position to be discharged through opening 36. Lighter material stirred .up by these scrapers will be carried up by the whirling air stream through passage l5 and thence returned into the mill.

The air stream forced or drawn upwardly through the mill must comprise a sufficient volume of air to carry the pulverized material in suspension, and at the same time this air stream must have sufficient velocity to carry the material upwardly toward the deflectors 33, and prevent pulverized material from falling downwardly through the annular air space l5. At the same time it is undesirable to utilize too great a, volume of air as, for example, when pulverized coal is carried directly to a furnace in suspension in this air stream. When there is only a light load in the mill, the volume of this air stream may be excessive for combustion purposes. It will be apparent that the volume of air can be cut down while maintaining the required velocity by simply narrowing the annular air passage l5, or restricting its upper outlet. However, this restricted volume of air will be insufficient to carry the pulverized material at those circumferential locations where the greater volume of material is discharged or spilled from the bowl. Also the annular passage will be of insuflicient width to permit large pieces of waste material, such as tramp iron, from falling through into the lower discharge chamber 35. According to the present invention those portions of the annular air space over which the outward discharge of material is relatively small (that is, outwardly from and adjacent the rollers) are substantially restricted so that only a ver limited volume of air flows upwardly at these locations. Those portions of the annular air space opposite the portions of the bowl between the rollers, where the greater portion of the material leaves the bowl, are left substantially unrestricted. In this manner the necessar air velocity is maintained with a greatly decreased volume of air, but the greater volume of the air stream is localized adjacent those segments of the bowl from which the majority of the material is discharged. At the same time, these portions of the passage are left of sufficient width to permit the downward discharge of large pieces of waste material.

In general, this may be accomplished by the use of a series of aligned arcuate horizontally extending restricting ribs positioned in the upper portion of passage I5. In the three-roller mill shown in Figs. 1 to 3 inclusive, three similar arcuate restricting ribs 40 are mounted within the casing A, for example by means of screw bolts 4| extending through the casing wall 3 and liner plates 1. These ribs 40 a e preferably of a substantially triangular cross section, the rear side being secured to the casing, and the top and bottom walls converging inwardly to a narrow inner edge portion 42 which is rather closely adjacent to the rotating bowl. There is a rather narrow space between the rib and the bowl for permitting a very restricted upward flow of air between the rib'and bowl. The'three ribs 4|! are positioned respectively back of the three rollers D, that is in radial alignment with the rollers from the vertical center line of the machine. Preferably the ribs 40 are of such length as to block off from onethird to one-half of the normal total outlet area of the annular passage 15. The spaces between the adjacent ends of ribs 40 are left substantially unrestricted, that is the normal full width of passage [5, at the positions opposite the spaces between adjacent rollers D. It is at these locations that the greater quantity of material is thrown or spilled from the bowl, and a full andsufficient volume of air at the requisite velocity will flow upwardl through these unrestricted spaces to handle the material thrown from the bowl. Large particles oi waste material can also pass freely downward through these unrestricted portions of the passage, and if any such waste particles should be thrown out above the ribs 40, these materials will work their way around until they fall through, the unrestricted passages. There will be sufficient air flow between the ribs and the bowl to take care of the small amount of pulverized material that is thrown out at these 10- cations and keep these restricted portions of the passageway clear. Preferably these ribs 40 are positioned in horizontal alignment adjacent the upper edge of the rotating bowl, or in alignment with the rotary deflector 34 as shown in Figs. 1 to 3 inclusive.

In the two-roller type of mill shown in Figs. 4, 5 and 6, the ribs 40 are made longer and only two of these ribs are used, one being positioned behind each roller. In this modification the ribs 40' are formed on arcuate liner plates 43 which are installed between sections of the usual liner plates 1. As shown in Fig. 4, each of the ribs 40 can be made in two or more sections, abutting end to end at 44. As shown in Fig. 6, the intermediate annular liner plates 45, betweenthe ends of the ribs 40' and opposite the unrestricted portions of the passageway may if desired be provided with narrow ribs 46 so as to partially cut down the air fiow at these locations. The width of these smaller ribs 46 may be proportioned as found necessary in accordance with the normal capacity of the mill and the necessary volume of air flow-.- Also, as shown in Figs. 4, 5 and 6, the restricting ribs may be positioned somewhat lower with respect to the upper edge of the bowl, although it is preferable to have this restricting means near the outlet of the annular passageway l5, thus localizing the greater volume of air flow at the locations between the rollers.

While this improvement was developed especially for use with a roller and bowl mill of the type herein. disclosed, it will be apparent that this improved air-flow controlling means could be used with other mills comprising a rotatable grinding ring, grinding elements cooperating with the inner portion of the ring, and an annular air passage between the outer portion of the ring and the enclosing casing through which passage flows the air stream for removing the pulverized material.

' We claim: v

1.' A mill comprising a closed casing, a grinding ring mounted within the casing for rotation about a central vertical axis, means for supporting and rotating the grinding ring, a plurality of grinding elements mounted to engage the grinding ring, means for introducing material to be ground between' the ring and grinding elements, there being an annular air passage between the grinding ring and the enclosing wall of the casing, means for forcing air upwardly through this annular passage to carry away pulverized material in suspension, and a plurality of arcuate, spaced apart ribs aligned end to end to form a horizontally extending circumferential series within the air passage, the ribs being positioned in those portions 'of the passage adjacent to and. radially beyond the grinding elements and functioning to restrict these portions of the annular passage and cause the greater portion of the air flow to pass through the unrestricted portions of the passage between the opposed ends of the ribs and opposite the spaces between the grinding elements.

2. A mill comprising a closed casing, a bowl centrally mounted within the casing for rotation about a vertical axis, means for rotating the bowl, means for introducing material to be ground into the bowl, a plurality of circumferentially spaced apart rollers mounted to engage inside the upwardly and outwardly extending wall of the bowl and grind the material between the bowl and rollers, there being an annular air passage between the outer wall of the bowl and the enclosing wall of the casing, means for forcing air upwardly through this annular passage to carry away pulverized material in suspension, and means for substantially restricting the annular passage at those locations adjacent and radially beyond the rollers so as to decrease the total volume of air flow without decreasing the velocity or volume of the upward air streams in theunrestricted portions of the passage adjacent and radially beyond the spaces between the rollers.

3. A mill comprising a closed casing, a bowl centrally mounted within the casing for rotation about a vertical axis, means for rotating the bowl, means for introducing material to be ground into the bowl, a plurality of circumferentially spaced apart rollers mounted to engage inside the upwardly and outwardly extending wall of the bowl and grind the material between the bowl and rollers, there being an annular air passage between the outer wall of the bowl and the enclosing wall of the casing, means for forcing air upwardly through this annular passage to carry away in suspension pulverized material, and a plurality of arcuate, spaced apart ribs aligned end to end to form a horizontally extending circumferential series within the air passage, the ribs being positioned in those portions of the passage adjacent to and radially beyond the rollers and functioning to restrict these portions 01 the annular passage and cause the greater portion of the air to pass through the unrestricted portions of the passage between the opposed ends of adjacent'ribs and opposite the spaces between the rollers.

4. Amill comprising a closed casing, a bowl centrally mounted within the casing for rotation about a vertical axis, means for rotating the bowl, means for introducing material to be ground into the bowl, a plurality of circumferentially spaced apart rollers mounted to engage inside the upwardly and outwardly extending wall of the bowl and grind the material between the bowl and r0llers,, there being an annular air passage between the outer wall of the bowl and the enclosing wall of the casing, meansfor forcing air upwardly through this annular passage to carry away in suspension pulverized material, and a plurality of arcuate, spaced apart ribs mounted on the inner wall of the casing and arranged in a horizontally extending circumferential series within the air passage, the ribs being located in those portions of the passage adjacent to and radially beyond the rollers and functioning to cause the greater portion of the air flow to pass through the unrestricted portions of the passage adjacent to and radially beyond the portions of the bowl between the rollers.

5. A mill comprising a closed casing, a bowl centrally mounted within the casing for rotation about a vertical axis, means for rotating the bowl, means for introducing material to be ground into the bowl, a plurality of circumferentially spaced apart rollers mounted to engage inside the upwardly and outwardly extending wall of the bowl and grind the material between the bowl and rollers, there being an annular air passage between the outer wall of the bowl and the enclosing wall of the casing, means for forcing air upwardly through this annular passage to carry away in suspension pulverized material, and a plurality of spaced apart passage-restricting means mounted on the inner wall of the easing substantially in horizontal alignment with the upper edge of the bowl, these restricting means being located in those portions of the passage adjacent to and radially beyond the rollers and functioning to cause the greater portion of the air flow to pass through the unrestricted portions of the passage adjacent to and radially beyond the portions of the bowl between the rollers.

6. A mill comprising a closed-casing, a bowl centrally mounted within the casing for rotation about a vertical axis, means for rotating the bowl, means for introducing material to be ground into the bowl, a plurality of circumferentially spaced apart rollers mounted to engage inside the upwardly and outwardly extending wall of the bowl and grind the material between the bowl and rollers, there being an annular air passage between the outer wall of the bowl and the enclosing wall of the casing, means for forcing air upwardly through this annular passage spaces between the rollers, the upper and lower wall of the bowl and grind the material between the bowl and rollers, there being an annular air passage between'the' outer wall of the bowl and the enclosing wall of the casing, means for forcing air upwardly through this annular passage to carry away in suspension pulverized material,

and a plurality of arcuate, spaced apart ribs mounted on the inner wall of the casing and arranged in a horizontally extending circumferential series within the air passage, the ribs being to carry away in suspension pulverized material,

and a plurality of arcuate, spaced apart ribs mounted on the inner wall of the casing and aligned end to end in a horizontally extending tion of the air flow to pass through the -unrestricted portions of the passage between the opposed ends of adjacent ribs and opposite the located in those portions of the passage adjacent to and radially, beyond the rollers and functioning to cause the greater portion of the air flow to pass through the unrestricted portions of the passage adjacent to and radially beyond the portions of'the bowl between the rollers, the upper and lower surfaces of the ribs converging toward an'inner edge portion located closely adjacent the outer surface of the bowl.

8. A mill comprising a closed casing, a bowl centrally mounted within the casing for rotation about a vertical axis, means for rotating the bowl, means for introducing material to be ground into the bowl, a'plurality of circumferentially spaced apart rollers mounted to engage inside the upwardly and outwardly extending wall of the bowl and grind the material between the bowl and rollers, there being an annular air passage between the outer wall of the bowl and the enclosing wall of the casing, means for forcing air upwardly through this annular passage to carry away in suspension pulverized material,

a plurality of adjoining arcuate liner plates cov-- ering the inner surface of that portion of the casing which surrounds the bowl, certain of these liner plates being formed with ribs which project in a horizontal circumferential series within the upper portion of the annular air passage adjacent the upper edge of the bowl, these ribs being positioned radially beyond the rollers so as to greatly restrict these portions of the passage and cause the greater portion of the air flow to pass through the unrestricted portions of the passage between the ends of aligned ribs.

WILFRED R. WOOD. KURT TOENSFELDT. 

